Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors, and turbines which have a rotating shaft and a sealing chamber adjacent the shaft wherein the mechanical seal prevents leakage of fluid from the sealing chamber. Many such mechanical seals include a pair of adjacent seal rings which have opposing seal faces that define a sealing region therebetween to sealingly separate the sealing chamber from an exterior region. Typically, one of the seal rings is mounted on the shaft so as to rotate therewith while the other stationary seal ring is non-rotatably mounted on a seal housing. Also, at least one of the rotating and stationary seal rings is axially movable. To maintain a seal between the opposed seal faces, the axially movable seal ring is axially loaded, such as by a spring or bellows, towards the other seal ring.
While the sealing region between the relatively rotatable seal faces defines the primary seal, secondary seals are provided between other adjacent components in the mechanical seal. For example, a secondary seal between the rotatable seal ring and the shaft or a shaft sleeve prevents migration of the sealed fluid therebetween, while a secondary seal between the stationary seal ring and a support element for the seal ring prevents migration of the sealed fluid between these components.
In spring biased seals, U.S. Pat. No. 5,813,674 discloses a non-bellows seal arrangement wherein a secondary seal between a seal ring and a seal ring holder is an annular gasket which has a U-shaped cup-like cross section and a spring disposed within the gasket to bias the gasket walls radially away from each other. Another seal arrangement having a spring energized plastic seal is disclosed in U.S. Pat. No. 6,116,610.
As such, there are a number of known annular gaskets available which use a spring actuated, pressure assisted cup design. These gaskets are typically machined from PTFE or plastic blended polymer based materials or Polyamid based resins that can be filled with graphite or other fillers to increase temperature or pressure limits. These materials are molded into bar form which resembles hard plastic at atmospheric temperatures.
U cup configured gaskets are machined from this molded bar, which gaskets are formed with a gasket groove and then require a spring to energize the thin cup lips or walls defined by the groove to form a seal when installed in an available cavity present in a mechanical seal. Normally these gasket shapes are designed to fit in a space approximately the size required for a standard cross section O ring. Spring designs which are fitted in the gasket groove to actuate the small machined cup can vary from; a wrapped formed ribbon material, an elliptical coil garter spring design, or as more commonly used, a small specially stamped and formed cantilever finger spring. Springs used to energize the U cups can be made from various metals for corrosion resistance or strength where high temperatures may otherwise yield the material.
These U cup gasket designs may be machined with horizontal opening U cups or vertically opening, radially inside or outside facing U cups. These small spring energized gaskets present an advantage when elastomers cannot be used due to extreme temperatures and/or corrosive environments. In some cases sliding friction or stiction between axially slidable seal components can be reduced in comparison to the use of elastomers which can swell, stick and deteriorate.
Parts can be machined from a wide variety of composite billet materials where pressure limits can be fairly high and cold flow and extrusion kept to a minimum. The gaskets are small in cross sections and provide their own sealing flexibility similar to that of a compressed O ring.
Typical construction of these self energized gaskets includes the machining of a cup shape to form the groove that is fitted with a spring and often opens toward the hydraulic pressure being sealed. Since the gasket is fully machined the cup can be machined horizontally or vertically. The spring installed in the cup shape provides initial sealing forces, wherein the gaskets are dimensioned to interfere with opposed sealing surfaces while the cup walls of the gasket can deflect to maintain contact with the opposing sealing surfaces. This sealing capability is enhanced as pressure is increased on the inside of the cup which thereby generates higher forces on the cup walls or lips to seal against the adjacent sealing surfaces. These designs are well suited for static sealing and hydraulic cylinder shaft sealing where actuator forces are high.
These known designs can be adequate for static services where both sealing surfaces of the cup walls abut against non-moving surfaces, even where hydraulic load applied to the inner cup surfaces in the groove is transferred to the adjoining sealing surface. However, tests have proven that as hydraulic pressure increases, the forces transmitted through the inner cup walls in contact with the abutting sealing surfaces increases. Hence the forces required to move one of the abutting surfaces in relation to the gasket sharply increases.
This calculation can be made by multiplying the gasket surface contact area by the hydraulic pressure times the coefficient of friction. At low product pressures, gasket surface loads are primarily being provided by the internal spring load. Under these conditions forces to move an abutting surface can be very low and reasonable for most applications where relative motion is required. The design principle of a machined U cup is to apply sealing forces equally to both sealing surfaces of the U cup, i.e., outside diameter to inside diameter, right to left etc.
Attempts to reduce sealing lip forces by relieving part of the gasket lip sealing surface and adding a support heal is less effective as pressures increase the deformation of the gasket.
As such, the application of these designs to mechanical seals for other than a static seal can pose critical problems. Using these known cup designs for a flexible rotating or stationary seal ring in a mechanical seal application can be problematic relative to maintaining seal face flatness and free axial movement of the seal ring relative to the seal sleeve. It becomes most noticeable when axial break out of friction forces at the gasket exceed the seal spring and hydraulic loading of the seal faces.
Hence, when known gaskets are used as a secondary seal particularly on a carbon structure mating seal ring, the internal forces of the gasket cup that are created initially by the spring and then further increased by hydraulic forces, apply a radial load relative to the carbon seal ring which acts radially on the seal ring. This force has a distortion effect on the lapped flat, sealing face and can cause excessive face heat and wear.
It is an object of the invention to provide an improved spring-energized gasket for use in mechanical seals as a secondary seal.
The gasket of the invention has special features which prove beneficial to applications in face type mechanical seals. In these seals, the seal rings can be made from very hard materials such as tungsten or silicon carbide, or softer materials such as mechanical grade carbon. The construction of the gasket is performed using a machining operation consistent with machining composite materials into various other configurations to form spring energized seals. The use of plastic compounds greatly reduces the stiction experienced with rubber compound gaskets.
Furthermore, the inventive gasket design is machined so as to be canted at a 45 degree angle relative to the diameter to be sealed. The new design configuration provides two features desirable to improve the performance of the gasket for use as a secondary gasket or seal for a mechanical seal face.
First, since the introduction of patterned or wavy face features on lapped mechanical seal faces, it is critical to remove external distortion effects on lapped sealing faces. Secondary gaskets that require squeeze or radial compression acting radially on the seal ring can put radial stresses on these faces which affect or alter the lapped configured sealing face. The canted self energized gasket of the invention, however, has a small vertically opening spring energized U cup preferably opening from its outside diameter. The gasket is captured in a groove so that the gasket is confined axially to apply just enough force to axially compress the outer gasket lip sufficiently to seal axially against the forward end of the groove. This seals the gasket to the seal ring without generating radial forces on the seal face.
Secondly, gasket sealing surfaces are flexible to move along the shaft or sleeve due to relative axial movement between the gasket and the equipment rotating shaft or other seal component. When this movement takes place, the secondary seal gasket of the seal ring slides on the component sealing surface to accommodate this axial movement. The gasket of the invention uses a canted, U shaped cup which is angled and spring energized to form a sealing lip which is biased against the opposing component surface. If the break-out friction of the gasket seal to the shaft, sleeve or other seal component is too high and exceeds the seal spring and hydraulic closing force, the mechanical seal faces may hang open with shaft motion causing excessive leakage and seal failure. The gasket has the canted, spring-energized U cup preferably located in the ID of the gasket, so as to seal the gasket to the shaft or sleeve or other seal component as necessary.
This gasket, however, does not apply radial forces through the mechanical seal face. Rather, the forward lip of the U cup is supported by an annular metal insert or support ring, preferably on the inside of the diameter of the gasket that supports the canted sealing lip except for the free end thereof which sealingly contacts the opposing component surface. The machined cup is designed with zero interference with the shaft or sleeve. Initial interference with the opposing seal component is achieved by the lip displacement caused by the spring which when installed holds the small lip in contact with the component surface. The support ring is machined to closely match the spring energized lip angle. As the gasket is installed over the sleeve, the lip angle changes and a slight gap exists between the support ring and the cup sealing lip. The lip thickness preferably is small, preferably less than 0.010″, so that the canted sealing lip will be flexible under pressure and mate up against the metal support ring. This in turn reduces the pressure caused load of the lip on the sleeve or shaft as the metal support ring or insert is supporting the vast majority of the load due to pressure. Only the free end of the sealing lip extends radially beyond the support ring for unsupported contact with the shaft or sleeve.
This design still provides the advantages of known gasket constructions. But also, the canted U cup design with support ring provides superior performance in achieving low breakout and sliding friction. As hydraulic pressures increase, the friction forces resisting shaft or sleeve axial movement are much lower than other known U cup design arrangement. The design and testing of this self energized U cup proved to be superior in performance to any other U cup configuration tested. Break-out friction at low and high pressures proved to be much lower, more than 50% less than known gaskets, and sliding friction was reduced by similar amounts.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.